Jökull - 01.12.1977, Page 52

TABLE 1 - TAFLA 1.
Tlie concentrations* (in ppm) of the major chemical components in the total discharge
of wells at Námafjall.
Styrkur (í ppm) aðalefna i heildarrennsli borhola i Námafjalli.
Component Well no. / Year and month of sampling
4 73.07 5 74.03 6 74.03 7 73.07 8 73.11 9 74.03
sío2 543 356 501 511 465 528
Na 133 128 132 122 124 119
Iv 23.2 19.4 23.1 22.6 21.7 22.2
Ca 1.3 2.2 0.7 0.7 1.3 0.9
Mg 0.09 0.04 0.04 0.11 0.03 0.03
COo** 110 113 158 257 131 184
so4 101 59 60 74 56 63
HoS*** 168 110 205 326 163 198
C1 16 23 25 19 22 18
F 1.5 0.8 0.8 1.5 1.0 0.9
h2 3.8 0.8 5.2 13.1 3.4 5.2
Diss. solids 990 719 931 974 861 913
V°C 255 218 248 251 241 254
* These concentrations have been calculated from analyses of water and steam samples collected on
the wellhead by assuming that the enthalpy of the total discharge corresponds to that of water
at the quartz equilibrium temperature (tqtz) given above.
#* Total carbonate.
*** Total sulphide.
result of pressure drop in tlie reservoir, caused
by withdrawal of fluid by the discarging wells.
Increased flashing in the reservoir, leads to cool-
ing of the reservoir water and thus upsets
temperature dependent solute/solute and min-
eral/solute chemical equilibria that govern the
water composition. Increased flashing in the re-
servoir may cause depletion in the volatile con-
tent of the well discharges and change the
ratios of individual volatile constituents, if that
part of the reservoir where the flashing occurs,
acts as an open system. Withdrawal of fluid in
excess of the natural recharge rate will upset
the balance between the extent of evapora-
tion and renewal of hot water front below,
and can in this way change the dissolved
solids content of the water feeding the wells.
Invasion of the surrounding cold ground water
50 JÖKULL 27. ÁR
into the geothermal system would have the same
effect.
The following discussion will only include
those chemical constituents, which have clearly
shown changes in their concentrations during
the observation period from 1970 to 1976.
Silica
A substantial lowering of the silica concentra-
tion of some of the well discharges has occurred
during the 6 year observation period. Other
well discharges have retained practically con-
stant silica contents. The lowering of silica is
strongest for wells 5 and 7. These wells are fed
by aquifers, which had measured temperatures
prior to exploitation nearest to the boiling
point curve (see Fig. 2). The decrease of silica